Monday, December 06, 2010

High BMI Predicts Early Death

Body Mass Index (BMI) has been getting a bad rap. That's somewhat deserving since, as a measurement that relies solely on height and weight, it lumps together as fat anyone whose weight is high relative to their height. So, if you have a lot of muscle it may say you are obese instead of finely chiseled. Conversely, if you have a "normal" weight relative to your height, it says you are "healthy." It doesn't get skinny fat.

Still, as a gross indicator of health BMI is useful (although its terminology is unfortunate, e.g. "morbidly obese"). This study draws out its use. When you're looking at 1.5 million people, the exceptions I noted above become blurred:

Body-Mass Index And Mortality Among 1.46 Million White Adults, New England Journal of Medicine, December 2, 2010.

The authors pooled data from 19 long-running studies - age range 19 to 84 years (median 58). A BMI of 22.5-24.9 was the reference category. Hazard ratios among women (men were similar) were:

1.47 for a BMI of 15.0 to 18.4

1.14 for a BMI of 18.5 to 19.9

1.00 for a BMI of 20.0 to 22.4

1.13 for a BMI of 25.0 to 29.9*

1.44 for a BMI of 30.0 to 34.9**

1.88 for a BMI of 35.0 to 39.9

2.51 for a BMI of 40.0 to 49.9

* For example, this is a 13% higher risk of death in overweight women.** And this is a 44% higher risk of death in obese women, etc.

This study found the healthiest BMIs were between 20 and 25.

Note that those who were underweight also died earlier than expected. Any hazard ratio above 1.00 designates increased risk.

Seniors have more of a problem with underweight than other age groups. Since they also have more comorbidities it may have been their age and not their weight working against them. But the authors adjusted for age. Lead study author, Amy Berrington de Gonzalez:

"By combining data on nearly 1.5 million participants from 19 studies we were able to evaluate a wide range of BMI levels and other characteristics that may influence the relationship between excess weight and risk of death.

Smoking and pre-existing illness or disease are strongly associated with the risk of death and with obesity. A paramount aspect of the study was our ability to minimize the impact of these factors by excluding those participants from the analysis."

Here's the Institute of Health's BMI calculator. I can't embed it but clicking it will take you to it.

I'd guess the women in these photos represent BMIs of around 18 or less, underweight according to this scale. What do you think? Both women appear tall. (Left: Angelina Jolie. Right: Carla Bruni.)

Here's a photo of Shakira that looks like a BMI of around 22 or 23, normal weight. I think she's short? 5'1" or 5'2"?

Here's one of Ronald Mike Tyson that I think shows how BMI fails as a dependable anthropometric tool. What do you say his BMI is here?